The biological lagoon systems in treating municipal wastewater mainly include facultative lagoons (non-aerated ponds) and aerated lagoons (partial mixing aerated ponds). Facultative lagoons do not have air input from an aeration system, which can be either a mechanical aerator system or a diffused bubble aeration system.
Facultative ponds are usually 4 to 8 feet in water depth with an aerobic layer overlying an anaerobic layer, often containing sludge deposits [Reference 1; all references are incorporated herein in their entirety]. They can be operated as controlled discharge lagoons or continuous discharge lagoon. The continuous discharge facultative lagoons typically are not permitted by regulatory agency in areas with very cold winter. The Iowa States Wastewater Design Standards requires that the detention time of one controlled discharge lagoon be at least 180 days. This requirement is to allow wastewater to be stored through cold winter and early spring. The effluent quality of continuous discharge facultative lagoon in winter cannot meet with water quality standards; especially to those streams with ammonia limits [2].
The concept of controlled discharge lagoons is to select a time of year when stream flow conditions are satisfactory for accepting a lagoon effluent discharge. In Mid-West states, a controlled discharge lagoon only allows to discharge in spring (April and May) and fall (October and November) [3]. The storage volume required for controlled discharge lagoons depends on the climates and water quality standards. If sufficient land is available, facultative ponds are a cost-effective means to provide wastewater treatment for municipal wastewater. Their operation is easy and their maintenance requirements are minimal. They are usually the most preferred system in hot climate zones [4].
Facultative pond design has been based upon biochemical oxygen demand removal; however, the majority of the suspended solids will be removed in the primary cell of a pond system. The BOD5 (biological oxygen demand) area loading rate recommended for an average winter air temperature of less than 0° C. is 10-20 lb/acre/day. The more extreme the environment is, the lower the loading rate is [5]. The facultative lagoon system typically has three cells. Two-cell facultative lagoon system only applies to very small installations. The three-cell lagoon system is operated in series and the four or more cell system can be operated in parallel or in series [6]. The wastewater enters into the first cell and flows through following lagoon cells.
The nitrogen removal from facultative ponds could be related to pH, detention time, temperature, and algae growth. When water temperature is below 8° C., algae die off and the biological activity in removal of nitrogen in facultative pond is limited [4]. Therefore, the ammonia removal in facultative ponds cannot reach good ammonia removal in the cold winter. In such cold climate zones, the spring drawdown of a controlled discharge lagoon may not provide discharge effluent in low ammonia concentration. In addition, the new water quality standards imposed by EPA in March 2006 have resulted in many streams in State of Iowa to receive much lower ammonia and E. coli limits than previously allowed. Accordingly, the design of a controlled discharge lagoon to meet low effluent ammonia limits in cold climate zones may not be appropriate.
In aerated ponds (partial mixing aerated ponds), oxygen is supplied mainly through mechanical or diffused air aeration rather than by photosynthesis and surface aeration. In the partial mix aerated pond system, no attempt is made to keep all of the solids in the aerated ponds suspended. Aeration serves only to provide oxygen transfer adequate to oxidize the BOD entering the pond. Many aerated ponds have evolved from overloaded facultative ponds that required aerator installation to increase oxygenation capacity. Aerated ponds are generally 6 to 20 feet in water depth with detention times of 3 to 10 days [7]. The number of cells used in series affects the detention time needed for treatment. In the State of Iowa, the minimum detention time for a three-cell aerated lagoon is 31 days [3]. This is because the very cold winter significantly reduces the biological activities in removal of BOD. The longer hydraulic detention time compensates the loss of biological activity.
Aerated lagoons continue discharging effluent at normal operating conditions. Minimum of three-cell aerated lagoon are required. The first two cells are aerated and the third cell is a quiescent cell to settle out solids and discharge effluent. When an aerated lagoon is designed right, the effluent will meet the secondary treatment standards in CBOD5 and total suspended solids (TSS) [2]. However, aerated lagoons are not designed to meet with stringent ammonia limits, especially in severe climate zones [3]. Aerated lagoon with insulated cover can retain wastewater temperature above 8° C. in winter [3]. With proper design, covered aerated lagoon is able to remove ammonia down to very low concentration in effluent. Several covered aerated lagoons constructed in the Cities of Villisca and Strawberry Point, Iowa have shown good ammonia removal all year round [3].